a) Field of the Invention
The present invention relates to a method for manufacturing filtration and distribution devices of improved structure for handing molten metals, especially molten aluminum.
The invention also relates to the molten metal filtration and distribution devices of improved structure that are so-obtained.
b) Brief Description of the Prior Art
During metal casting, especially aluminum casting, it is of common practice to use a bag made from a heat-resistant fabric, like a glass fiber fabric, in order to filtrate the molten metal that is being poured through a spout, and to distribute it all over the mold surface. Of course, the shape and dimensions of the bag depend on the molding technology and the casting parameters. Presently, there are mainly two different types of bags used in the metallurgical industry.
The first type of bags include bags of large sizes that are made of a fine mesh glass fiber fabric. These bags are often called "channel bags" and a typical example thereof is shown in FIG. 1 of the accompanying drawings. As can be seen, the illustrated channel bag 1 is in the form of a rectangular pan 3 which is made of an open weave heat-resistant fabric, preferably an open weave fabric made of glass fibers, whose purpose is to let the molten metal flow down into the mold. The pan 3 is provided with reinforcing bands or areas 5 made of a dense heat-resistant fabric. These reinforcing areas 5 extend over the bottom and some portions of the sides of the pan and gives extra-strength and rigidity to the bottom of the pan. The bag can be attached under the pouring spot with a set of rods inserted into hems 9 provided for this purpose on the lateral sides of the pan 3.
The second type of bags used in the industry include bags of much more complex structure, which are made of several parts of fiberglass fabrics sewn to each other. These bags called "combo bags" in the industry, are usually smaller in dimensions than the channel bags and mostly used with automatic casting control. A typical example of combo bag is shown is illustrated in FIGS. 2 to 6 of the accompanying drawings. As can be seen, the illustrated combo bag 11 which can be 13" long, 5" wide and 4" high, comprises a main body 13, an outer shell 15, end patches 17, bottom and reinforcement patches 19, and positioning tabs 21 all made of heat-resistant fabrics, preferably fiberglass fabrics. All these parts of different shapes and functions and are sewn together as will now be described in great detail.
The main body 13 is better shown in FIG. 3. It consists of a rectangular box with an open top made of an open-weave glass fiber fabric. The size of the openings of the fabric can vary depending on the weave construction and the type of yarns used. This fabric controls the distribution, the spread and the speed of metal around the mold in order to get the best temperature profile.
The outer shell 15 is shown in assembled position onto the main body 13 in FIG. 4. It consists of a piece of a solid glass fiber fabric which is designed to control the direction of the metal flow, retain oxides and reduce turbulence associated by the change in flow direction from vertical to horizontal. It is sewn to the main body 13 along the length of the sides and bottom thereof, at mid-height and width, respectively (see the sewing lines 16).
The end patches 17 are shown in assembled position onto the main body 13 in FIG. 5. They also consist of pieces a solid fiberglass fabric sewn in the top portions of the side ends of the main body 13. The main role of those patches is to keep the oxides generated by the turbulence inside the bag. As aforesaid, the patches are located at the ends of the bag and are high enough to direct the flow below the surface level of the metal poured into the combo bag.
The bottom and reinforcement patches 19 are also shown in assembled position in FIG. 5 (only one of them is actually shown, in dotted lines). They consist of pieces of a dense solid glass fiber fabric. These patches 19 are fixed to the bottom of the main body 13 or of the outer shell 15. Their length can be variable depending on the desired stiffness. They act to spread the metal sideways, to protect the filtering fabric of the main body 13 from burn through, to minimize the bag deformation under the metal pressure at the cast start and to avoid vertical diffusion.
As is shown in FIG. 6, bottom holes and side windows 20 are provided in the outer shell 15 to control the flow of metal by minimizing hot spots around the mold. These windows 20 also help to drain the bag at the end of the cast.
Last of all, the positioning tabs 21 are shown in FIG. 2. They consist of pieces of thin and dense glass fiber fabric that extend transversally across the open top of the main body 13 of the bag to correctly locate the bag around the pouring spout.
All those different parts 13, 15, 17, 19 and 21 are sewn together in a certain order to optimize the production time and to reduce the production costs.
Before sewing, a coating is usually applied onto the fabric to improve its chemical resistance to the molten metal and to increase the stiffness of the bag. Such an Increase in stiffness is important to maintain an even flow and a good molten metal temperature profile during the cast.
There are many different coatings that are used in the industry PVA, ceramic and phenolic resin are good examples. All these known coatings have an organic part which burns off at the molten metal temperature. Sometimes, the generated smoke is not toxic but in certain cases, depending on the coating composition, such a smoke may be toxic.
As can be understood, the construction of the open weave and solid fabrics used to make the bag as well as the type of yarns used to weave these fabrics, have a substantive influence on the rigidity, the chemical resistance to molten aluminium and the metal flow distribution. Without going in too much details, the characteristics that are generally wanted for the combo bags are:
a sufficient stiffness to ensure an even molten metal flow; PA1 a good chemical resistance to the molten metal, especially molten aluminum and high magnesium alloys; PA1 a certain amount of deformation to accommodate the change in flow direction; PA1 a smokeless and non-toxic behaviour at molten metal temperature; and PA1 a non-expensive cost.
As it can now be appreciated, the bags used in the metallurgical industry, and more particularly the combo bags, are of very complex structure and call for many different parts and a very specific selection of fabrics and finishes.